Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
FIG. 1 depicts an example of a transmitter 100 for wireless communications. The analog section is highlighted. Modern wireless standards are based on digital modulations in complex domain therefore the desired signal is achieved by the quadrature combination of in phase (I) and quadrature (Q) signals. A transmitter digital signal processor (DSP) 102 outputs are fed into the analog upconversion chain through digital-to-analog converters (DACs) 104. Analog low pass filters LPF 106 (often called reconstruct filters) cancel out the unwanted DAC signal replicas. Gm buffers 108 act as driver for the mixer transconductor.
Gm buffers 108 receive I and Q signals from low pass filters (LPFs) 106 and drive the upconverter transconductor stage. Upconverters are based on the classical Gilbert cell therefore they include a base-band current input stage (transconductor) 112, a mixing stage (often called quad) 114 and the RF output section 116. Mixing stage 114 multiplies the incoming base-band current by a local oscillator signal. A synthesizer generates a local oscillator (LO) signal. Frequency divider/LO generator then generates the I version for the LO signal (LO I) and Q version for the LO signal (LO Q). The LO I and LO Q signals may be differential. Mixers use the LO I signal and the LO Q signal to upconvert the baseband I and Q signals to differential radio frequency (RF) signals.
Mixer RF output stage may have a resonant load. In this example, a resonant transformer—balun (BALanced-UNbalanced) 116—converts the incoming differential RF signal into a single ended signal. The output signal is sent to a power amplifier that is off an integrated circuit (IC). Additionally, balun 116 may be off chip or in the package of the preamplifier (PA). More in general transmit path may be multimode/multiband and can transmit signals over multiple wireless bands. Wireless bands may correspond to different wireless standards or different operative bands defined for the same standard. In such environment, several baluns 116 could be provided for different wireless bands, such as a second generation (2G) high band (HB), a 2G low band (LB), a third/fourth generation (3G or 4G) low band and high bands. Similarly in a multi standard wireless equipment outputs could be WLAN 2.4G or 5G, Bluetooth, etc. A balun 116 is selected based on which wireless band is used. Accordingly, transmitter 100 offers a direct up and out approach that does not include a PA buffer. That is, balun 116 directly outputs the RF signal off the chip without going through a PA buffer. Thus, no additional noise and distortion is added from the PA buffer.
The same architecture for transmitter 100 can be used to drive multiple wireless bands. For example, some bands may require differential outputs and some may require single-ended outputs (balanced or unbalanced outputs). In one embodiment, a single-ended output may be converted to multiple differential outputs with a package change. That is, the single-ended output may be converted by changing a ground from a terminal of balun to output a second signal. This outputs differential signals from the two terminals of balun 116. Thus, the same architecture can be configured to drive balanced or unbalanced outputs.
FIG. 2 depicts an example of a Gm stage for transconductor mixer driving. Nodes IN+ and IN− are connected the low pass filter (LPF) differential outputs. The input resistances Rin convert the LPF output voltage signal into current. The Gm stage close configuration guarantees low distortion in the voltage-to-current conversion. Gm buffer 108 is here schematically depicted as a dual stage amplifier; however, more than two stages could be used for large signal bandwidth or high dynamic range performance. A first stage (input stage) guarantees gain and low noise performance while the second stage (output stage) is used for reference current generation. In order to achieve a correct mixer transconductor driving the Gm output stage is a replica or scaled replica (if current gain is required) of the mixer base-band input stage. From a behavioral point of view, the Gm stage acts as a current mirror reference (closed in a feedback loop) for the mixer transconductor.
A conventional approach is a class A Gm output stage as depicted in the FIG. 2. In order to cover the input stage dynamic range, the output stage bias current (Ibias) should be larger than the peak of the incoming input signal. Modern communication standards are moving towards high data rate modulation scheme. The resulting transmission signals present a very high ratio between root mean square and instant peaks (called peak-to-average-ratio or PAR). For class A blocks, high PAR signal handling is very inefficient (bias current should be sized based on signal peaks) resulting in power waste.